The present invention relates to a display controller suitable to a display of an information processing system, and more particularly to a display controller to be used when a representation of a certain number of pixels is to be displayed on a display which can display a representation of a larger number of pixels.
Demand for portable personal computers such as notebook-type personal computers and laptop-type personal computers having flat panel displays has been increasing to exceed that for desktop-type personal computers because they are easy to carry and can be battery-powered.
Recent trend is the increase of the number of pixels for display in order to display more information on one screen, and demand for a flat panel display having a large number of pixels has been increasing.
As a result, a display controller which enables a display having a fixed large number of pixels to also display a smaller number of pixels required by existing application software has been required, and various techniques therefor have been proposed.
For example, in a display disclosed in JP-A-1-98793, a start of display position is stored in an internal register of a display controller as an offset value so that information is displayed at a center of the display screen. For example, as shown in FIG. 4, when a vertical number 2 of pixels of the display 1 is 480 dots and a vertical member 3 of pixels required by an existing application software is 400 dots, the offset value 4 for the start of display position is set to "40" to start the display from the start of display position 5 so that the information is displayed at the center of the display screen.
In a display disclosed in JP-A-59-61874, the same information is continuously displayed at every given number of lines so that the information is additionally displayed by the increased number of pixels. For example, as shown in FIG. 5, when the vertical number 2 of pixels of the display 1 and the vertical number 3 of pixels required by the existing application software are 480 dots and 400 dots, respectively, as they are in the above-mentioned center display system, the same data is displayed in two lines every five lines so that the display area is expanded to the entire display area of the display 1.
In order to switch among a plurality of display modes including the two display modes described above, the prior art system uses the content of an I/O register which a central processing unit (CPU) sets by software. This is explained below with reference to the drawing.
FIG. 3 shows a prior art information processing system having a function of switching among a plurality of display modes including the screen center display mode and the incremental pixel supplement display mode described above.
Numeral 1 denotes a dot matrix display, numeral 100 denotes a display control LSI for controlling the display, numeral 101 denotes a display memory for storing display data, numeral 15 denotes an address generator for generating a read address when the display data stored in the display memory 101 is displayed, numeral 14 denotes a data converter for converting the read display data while interfacing it with the dot matrix display 1, numeral 102 denotes a display switching register in which a value corresponding to the display mode to be selected from the plurality of display modes including the two display modes described above are set, numeral 104 denotes a keyboard, numeral 103 denotes a keyboard controller for controlling the keyboard 104, and numeral 105 denotes a CPU for processing application software and display control software.
In order to select one of the plurality of display modes including the screen center display mode and the incremental pixel supplement display mode described above, an operator depresses a specified key of the keyboard 104 to designate a desired display mode. When the specified key is depressed, the keyboard controller 103 issues an interrupt request to the CPU 105, which sets a value corresponding to the designated display mode in the display switching register 102 in an interruption processing routine. The address generator 15 generates a read address to be supplied to the display memory 101 so that the display mode corresponding to the value set in the display switching register 102 is selected.
When a representation of a small number of pixels required by the conventional application software is to be displayed on a display which is capable of displaying with a larger number of pixels, it is preferable for the operator to determine a desired one of the plurality of display modes including the screen center display mode and the incremental pixel supplement display mode while the operator watches the screen of the application software.
However, depending on the application software, the interruption may be inhibited or a keyboard processing routine is governed by the application software and any key entry which the application software does not use may be ignored.
In such a case, the prior art interruption processing system cannot switch the display mode during the execution of the application software.